A facsimile apparatus which sends image data converted to HDLC format and has an error correction function using full duplex communication is known as follows.
A transmitting station converts encoded image data to HDLC format for every predetermined number of bytes, e.g., 512 bytes, 256 bytes, 64 bytes, etc. and sends converted image data to a receiving station. Each group of data of every predetermined number of bytes is called a frame of data. Each frame has an identification (ID) number, for example a module of three bits used as the ID number. Therefore, each frame has as its ID number one of the numbers 0 through 7. If the communication system has a long delay time or if a data amount of each frame is small, the number of bits for the module increases. For example, a module of eight bits is then used, and each frame has one of the ID numbers 0 through 255.
The receiving station checks the CRC and checks the continuity of frame numbers showing that frames have been received correctly. The receiving apparatus returns a signal NACK, which is a tonal signal having a predetermined band width which is outside of that of the image signal, or the like to the transmitting station when the receiving station detects any errors.
When the transmitting station receives and detects the signal NACK, the transmitting station stops sending image data.
The receiving apparatus sends the ID number of an error frame in which any errors occurred with a FSK signal of 300 bits per second to the transmitting station, and the transmitting station resends image data of the error frame designated by the receiving apparatus.
In the above-mentioned error correction, however, the transmitting station has to stop transmission every time the receiving station detects any error, and has to resend image data from the frame in which the error occurred, so that when an error occurs frequently it takes a long time to send image data.
Avoiding this problem, a half duplex communication method has been proposed which has an error correction function sending image data converted to HDLC format. This method operates as follows.
Image data having a plurality of frames is provided as one block, and both a transmitting station and a receiving station store the image data of each block. After the receiving station receives a procedure signal showing a completion of sending one block, the receiving station returns the ID numbers of the error frames in which errors occurred as a request signal for retransmission. The transmitting station retransmits the image data corresponding to the error frames in response to the request signal for retransmission.
In this method, however, using the half duplex communication method, after the completion of sending one block of image data, the receiving station informs the transmitting station of the ID numbers of error frames by a procedure signal which is relatively slow, and the transmitting station sends the image data again. Therefore, even if there is only one error frame, it takes three seconds to send the procedure signal and it takes one second for retransmission. Thus the total transmission time is longer even though there is only one error frame.